1. Field of the Invention
The present invention is related to semiconductor materials, methods, and devices, and more particularly, to a lateral growth method for defect reduction of semipolar nitride films.
2. Description of the Related Art
Gallium nitride (GaN) and its alloys with indium (In) and aluminum (Al) (referred to as (Al, In, Ga)N or nitrides) are currently used to produce visible and ultraviolet optoelectronic devices and high power electronic devices. Nitride films are grown heteroepitaxially by techniques such as metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE). Nitride light emitting diodes (LEDs) and laser diodes (LDs) are currently commercially available.
The most stable structure of nitrides is the hexagonal würtzite structure. In würtzite, there is a unique c-axis, and two or three a-axes, depending on choice of unit cell. This phase consists of alternate stacking of cation and anion planes in the c-direction. The planes have hexagonal symmetry and have a stacking sequence of AaBb along this c-axis, where upper case represents anions and lowercase represents cations, and the letters represent the stacking sequence along the c-direction. The stacking sequence of AaBb, i.e., A (11) a (12) B (13) b (14), is shown in FIG. 1, viewed perpendicular to the c-axis.
Current state of the art group III-nitride devices are grown in the c-plane orientation. However, the symmetry of the würtzite structure dictates that there will be a net polarization vector normal to the c-plane (i.e., along the c-direction). This polarization is detrimental to the performance of optoelectronic devices, as it causes band bending and an effect known as the quantum confined stark effect in quantum wells. The most important results of this are decreased radiative recombination efficiency, red shifted emission, and blue shifting of the emission with increasing drive current. The decreased recombination efficiency results from the spatial separation of the electron and hole wave functions. The red shift in emission is due to band bending, and the emission blue shifts with increasing drive current as the applied field overcomes the built-in polarization fields.
The total polarization is a sum of the spontaneous and piezoelectric polarization. The spontaneous polarization is an intrinsic property of the crystal and depends only on the composition of the nitride alloy. The piezoelectric polarization is a result of strain experienced by the lattice. There is usually strain in heterostructures such as InGaN quantum wells (QWs) on GaN, as layers of different composition in nitride heterostructures generally have different lattice constants from one another. The piezoelectric polarization increases with increasing strain, thus the polarization increases as the In composition is increased in InGaN/GaN QWs. This effect has made the fabrication of green LEDs very difficult and the fabrication of green LDs virtually impossible for current c-plane orientation nitrides.
The nitrides do not lend themselves to bulk crystal growth for several reasons. First, GaN is a refractory material and must be synthesized at elevated temperatures. Second, a very high over-pressure of nitrogen is required to prevent GaN from decomposing at elevated temperatures. Third, the high bond strength of the N2 molecule complicates nitrogen incorporation into GaN crystals. As bulk GaN crystals are not widely available, current devices are grown on foreign substrates heteroepitaxially. The nature of heteroepitaxial growth leads to significant defect densities, most prominently in the form of threading dislocations. Researchers are continually trying to reduce defect density as defects act as nonradiative recombination centers. In c-plane nitride growth, as well as other semiconductor materials systems, the threading dislocation defects predominantly propagate along the principal growth direction. As such, laterally growing polar and nonpolar nitrides tend to exhibit reduced defect densities. However, defect propagation in semipolar films is as yet unclear and will be important to future research and development efforts.